FATTY SUBSTANCES. (GENERAL METHODS) 393
tilling the solvent, drying the residue on a water-bath and powdering it.
Of this resinate, 0-2 gram is heated in a water-bath with 10 grams of the
oil to be tested until completely dissolved, i gram of precipitated silica
and a glass stirring-rod are placed in a dish and the whole tared, 1-02 gram of
the oil plus resinate being aUowed to f aU drop by drop on to the sand and the
whole weighed. The mass is well mixed and left exposed to the air at the
temperature 17-25° for drying oils or 20-30° for other oils. After 6 hours
and after further successive intervals of 12 hours the basin is weighed (the
mass being stirred each time) until of constant weight. The maximal
increase of weight, multiplied by 100, Bishop terms the degree oj oxidation
of the oil.
Drying oils usually absorb oxygen easily and rapidly, so that after an expo-
sure of 2-3 days the absorption is practically at the maximum attainable even
after 8-10 days. On the other hand, non-drying oils do not increase in weight
during the first days of exposure and begin to absorb a small quantity of oxygen
only after 5-6 days.
According to Livache, the maximum amount of oxygen absorbed per 100
parts of linseed oil is about 14, the amounts for walnut oil, poppyseed oil, cotton-
seed oil and beechnut oil being 8-5. Olive, arachis, sesame and colza oils absorb
only 1-3% of oxygen.
According to Bishop's method, the mean degree of oxidation is 17 for lin-
seed oil, 13-15 for hempseed, poppyseed and walnut oils, and 6-9 for cotton-
seed, sesame and arachis oils.
23. Colour Reactions.
Different fatty substances, more particularly the fatty oils, give special
colorations with various reagents, such as acids, alkalies and different salts.
Some of these reactions serve to distinguish certain groups of oils, whilst
others, being specific for a single oil, serve to characterise the latter. These
specific reactions will be dealt with in the special part in the paragraphs
treating of the particular oils (see Cottonseed Oil, Sesame Oil). Some of
the group reactions in more general use for the distinction of the different
groups of vegetable oils (for animal oils, see Fish Oils) will be described
1. HEYDENREICH'S REACTION. Five or six drops of the oil are allowed
to drop from a pipette on to about 5 c.c. of pure sulphuric acid (66° Baume)
in a flat-bottomed porcelain dish. In about three minutes the oil spreads
to form a very thin layer on the acid; the colour formed during this time
in the zone of contact between oil and acid is observed.
With ohve, arachis and almond oils, there is no sensible change of colour,
the oil remaining pale yellow or yellow, although sometimes with olive oil
a greenish-yellow coloration appears. With very old or rancid oils, colours
tending to orange or brown may be formed.
Semi-drying oils give orange or brown colorations, and drying oils brown
or black colorations, while the oil forms a thick skin (see also under the
2. HAUCHECORNE'S REACTION. 6 c.c. of the oil are vigorously shaken
in a test-tube with 2 c.c. of nitric acid prepared from 3 vols. of pure nitric
acid of 40° Baume and i vol. of water. Note is made of the coloratione liquid fatty acids are (i) those of David2 and Fakiola* based on